Movie: Some MP4s won't play online

[TEParsons]

Issue originally raised on PsychoPy repo, but concerns PsychoJS. Original text:

Hello everyone,
I am having some issues to load movie stimuli to Pavlovia.
I have created an experiment where subjects needs to see a video clip (2 minutes) right before the task. It works pretty well offline, but once I load it in Pavlovia, the movie does not start, showing only the first image for the whole duration of the video stimulus, without audio. Stimuli are .mp4, saved in the moviepy backend, last for 120 s and are 25 to 100 MB.
Does any one else had the same problems?

Here's the link to the task:
https://pavlovia.org/Stefano_PhD/thai_coam

Any suggestions?
Thanks to everyone

psychopy/psychopy#4235